Multi-Isotope Theory Validation of Calorimetric Electron Capture Spectroscopy*

High statistics, high resolution calorimetric spectroscopy of electron capture in ¹⁶³Ho using microcalorimeters has the potential to probe the neutrino kinematic mass with sub-eV mass sensitivity, but the theoretical description of the spectrum must be well-understood to determine the neutrino kinematic mass without bias or significant systematic uncertainty.  We have recast the theoretical description in terms of four key categories: choice of atomic code, description of atomic-nucleus overlap, truncation of the overlap of two fully-antisymmetrized atomic wave functions, and complexity of atomic excitations considered. This decision-space is mapped out with predictions made for comparison to experimental observables. Spectra have been acquired with transition edge sensor microcalorimeters with 7–35 eV FWHM depending on source-absorber matrix properties. Multi-isotope cross validation experiments with ¹⁶³Ho, ¹⁹³Pt, and ⁵⁵Fe and comparison to theoretical calculations are used to determine the level of fidelity needed in the calculations.